Content De-duplication for CDNi Optimization
draft-jin-cdni-content-deduplication-optimization-01
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| Authors | WeiYi Jin , Mian Li , Bhumip Khasnabish | ||
| Last updated | 2012-06-18 | ||
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draft-jin-cdni-content-deduplication-optimization-01
CDNI W. Jin
Internet-Draft M. Li
Intended status: Informational B. Khasnabish
Expires: December 21, 2012 ZTE Corporation
June 19, 2012
Content De-duplication for CDNi Optimization
draft-jin-cdni-content-deduplication-optimization-01
Abstract
Recent explosive growth of content delivery/distribution networks
(CDNs) and their interconnection are causing unintended repetition of
content storage in the same dCDN. This can be avoided by using a
suitable de-duplication mechanism. This document explores the
scenarios which create the problems, and then discusses the
approaches to eliminate the duplicated transmission of the same
content from uCDN(s) to dCDN in CDNi networks. To implement the
optimization, some enhancements to the CDNi metadata model and
interface are required.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on December 21, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 4
2.1. Scenario 1 . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Scenario 2 . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Scenario 3 . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Content Naming for CDNi . . . . . . . . . . . . . . . . . . . 7
3.1. Uniqueness . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Ownership . . . . . . . . . . . . . . . . . . . . . . . . 8
4. CDNi Content De-duplication Optimization Implementation . . . 8
4.1. Constant URL . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Content Naming Mechanism . . . . . . . . . . . . . . . . . 9
4.3. Description of Content De-duplication . . . . . . . . . . 10
4.3.1. Pre-Positioned Content Acquisition . . . . . . . . . . 10
4.3.2. Dynamic Content Acquisition . . . . . . . . . . . . . 12
4.3.3. Content Removal . . . . . . . . . . . . . . . . . . . 14
5. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Normative References . . . . . . . . . . . . . . . . . . . 16
8.2. Informative References . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
In some CDNi deployment, the dCDN occasionally caches the same
content copy multiple times from the same Content Service Provider
(CSP). For example, the CSP may have the agreement with two
authoritative CDNs, and both could be the upstream CDNs of the same
dCDN. Cascading of CDNs may result in a similar scenario as well.
The top-layer uCDN establishes connections with two intermediate-
layer uCDNs respectively, and both connect to the same bottom-layer
dCDN. In such scenarios, the dCDN may receive the content via 'push'
from one of the uCDN via pre-position procedure, and then may also
request for downloading the same content from another uCDN via
another pre-position procedure or upon user's content request.
Storing the same content multiple times not only wastes the dCDN's
the memory or storage resources, it also causes wastage of
transmission bandwidth that is used to deliver the same content
repeatedly. Therefore, it is necessary to avoid delivering the same
content from different uCDNs to dCDN repeatedly. In this draft, we
list a set of scenarios which may cause repeated delivery of the same
content. A feasible solution for content de-duplication is then
discussed.
In order to address the content repetition problem, several issues
need to be considered.
* How to detect content repetition by dCDN.
* How to avoid content repetition, when one or more uCDNs selects one
dCDN to deliver the same content to multiple User Agents.
This document provides detailed analysis on the issues of content
repetition. We realize that there is a need to develop an optimized
mechanism to de-duplicate the content in CDNi network. In order to
implement such optimization, enhancement to CDNi metadata model and
interface may be required.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
This document reuses the terminology defined in:
[draft-ietf-cdni-problem-statement-06],
[draft-ietf-cdni-requirements-03],
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[draft-ietf-cdni-framework-00], and
[draft-ietf-cdni-use-cases-07].
Resource Id: a metadata object (e.g., partial or whole URL, or other
format) which is generated by uCDN and identifies the storage of the
content in the uCDN.
Content Id: a metadata object (e.g. a URN) which uniquely identifies
the content in the scope of CDNi.
2. Deployment Scenarios
This section illustrates several CDNi deployment scenarios that
typically lead to duplicated content in the same server.
2.1. Scenario 1
As depicted in Figure 1, two interconnected CDNs - CDN-A(uCDN) and
CDN-B(dCDN) - both have contracts with CSP. CDN-B plays two roles at
the same time: downstream CDN of CDN-A and Authoritative CDN of CSP.
When an end-user of CDN-A initiates content request from the CSP,
CDN-A decides that CDN-B should be the serving CDN. Then CDN-A
redirects the request to CDN-B. If CDN B does not have a local copy
of the requested content yet (cache miss), CDN B ingests the content
from CDN A. Normally CDN-B, as Authoritative CDN, is very likely to
have already cached this content from original server. If CDN-B
cannot identify the requested contents as the same content, this same
content will be repeatedly retrieved and cached.
As the location of the content in a CDN is normally assigned by CDN
itself, the URLs of the same content are likely different between
CDNs. So it is not enough to determine whether the content to be
retrieved and cached is the same only by the URL of the content.
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+-------+
| CSP |
+-------+
/ \
,--,--,--./ \,--,--,--.
,-' `-. ,-' `-.
( CDN Provider A )=====( CDN Provider B )
`-. (CDN-A) ,-' `-. (CDN-B) ,-'
`--'--'--' `--'--'--'
|
+------------+
| User Agent |
+------------+
=== CDN Interconnect
Figure 1 Interconnected CDNs with the same CSP
2.2. Scenario 2
As depicted in Figure 2, both CDN-A and CDN-B establish
interconnections with CDN-C that acts as a dCDN. Thus, CDN-C will
cache the content for CDN-A and CDN-B. When both CDN provider A and
CDN provider B have agreements with the same CSP for content
delivery, CDN-C may be required by CDN-A and CDN-B separately to
retrieve and cache the same content from the CSP. Similar to
Scenario 1, CDN-C is also likely to suffer from content repetition
troubles.
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+-------+
| CSP |
+-------+
/ \
,--,--,--./ \,--,--,--.
,-' `-. ,-' `-.
( CDN Provider A ) ( CDN Provider B )
`-. (CDN-A) ,-' `-. (CDN-B) ,-'
`--'--'--' `--'--'--'
\\ //
\\,--,--,--.//
,-' `-.
( CDN Provider C )
`-. (CDN-C) ,-'
`--'--'--'
|
+------------+
| User Agent |
+------------+
=== CDN Interconnect
Figure 2 Interconnected CDNs with the same CSP and with one dCDN
2.3. Scenario 3
Now we consider the case of cascaded CDNs, as depicted in Figure 3.
Note that the top-layer Upstream CDN-A has direct contract with CSP
and interconnects with two middle-layer CDNs (CDN-B and CDN-C) that
have the same bottom-layer Downstream CDN (CDN-D) interconnected to
them. Consequently, there are two possible delivery paths for CDN-D
to cache the contents of CSP, one is CDN-A -> CDN-B -> CDN-D, and the
other is CDN-A -> CDN-C -> CDN-D. CDN-D may need to cache the same
content by upstream CDNs(CDN-B and CDN-C) on different paths. If the
URL of the content is changed by CDN-B or CDN-C, CDN-D cannot be
aware of the contents to be cached and therefore this may lead to
storing of duplicated contents.
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,--,--,--.
,-' `-. +-------+
( CDN Provider A )----| CSP |
`-. (CDN-A) ,-' +-------+
//-'--'-\\
,--,--,--.// \\,--,--,--.
,-' `-. ,-' `-.
( CDN Provider B ) ( CDN Provider C )
`-. (CDN-B) ,-' `-. (CDN-C) ,-'
`--'--'--' `--'--'--'
\\ //
\\,--,--,--.//
,-' `-.
( CDN Provider D )
`-. (CDN-D) ,-'
`--'--'--'
|
+------------+
| User Agent |
+------------+
=== CDN Interconnect
Figure 3 Cascaded CDNs
3. Content Naming for CDNi
It is well known that CDNs have their own content naming mechanisms,
most of which are independent and separated from one another due to
the use of different algorithms such as Hash algorithms. It implies
that for the same content distributed by two CDNs, the corresponding
content identifiers are likely to be quite different.
[draft-ietf-cdni-requirements-03] treats the information regarding
CDN content naming as intra-CDN information and the CDNI solution
MUST not require intra-CDN information to be exposed to other CDNs
for effective and efficient delivery of the content. Therefore,
establishing a uniform content naming mechanism is urgently needed
for CDNi network. This mechanism which can be implemented by CDNI
Metadata Distribution Protocol may have the following properties.
3.1. Uniqueness
CDNi content naming mechanism must guarantee the uniqueness of the
content identification. Although URL is widely used for identifying
network resource, it is not quite suitable for content identification
in CDNi network where content de-duplication needs to be taken into
consideration. Although the method of URL match is commonly used by
many cache systems to detect the repetitive files with same name for
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avoiding content repetition, it will probably fail for CDNi case.
This is due to the fact that different forwarding mechanisms are used
in different CDNs involved. The user-originated requests are always
snooped by the DPI (deep packet inspection) devices before
transmitted to the original server, whereas the requests received by
dCDN are always redirected by one or more uCDNs. Since there is no
guarantee that the URLs will not be changed through the redirection
process, a new type of object needs to be defined to represent the
uniqueness of content identifier.
For the CSP's the contents that are distributed into different
interconnected CDNs, the related metadata objects may be somewhat
different in many cases. For example, in Figure 2, when both CDN-A
and CDN-B delegate the delivery of the same CSP's content, the
content metadata such as (a) content description, (b) access policy,
and (c) security policy may be not be exactly the same in both cases.
Such metadata information is not suitable for content identification.
Consequently, we need to define a new type of metadata object that
helps uniquely identify the same content.
3.2. Ownership
CDNi content naming mechanism should embody the ownership of content
identification. Typically, a CDN provider may have contracts with
many CSPs for delivering their contents, as well as operate its own
Content delivery network. However, it may happen that a lot of
contents published by these CSPs may be very similar, and many of
them may even be exactly the same. Therefore, the problem is whether
these identical copies are from the same CSP or how the
interconnected CDNs can verify that these contents are identical.
(Note: Such copies are pointed by the same content identification
only if they are from the same content source.) For a traditional
(non-interconnected) CDN, there is no problem to distinguish them via
its intra content naming mechanism. When a CDN interconnects with
other CDNs, the condition becomes more complicated due to the lack of
awareness of CSP's content when the CDN acts as a dCDN.
4. CDNi Content De-duplication Optimization Implementation
4.1. Constant URL
In general, URL-based mechanism can be used to implement content de-
duplication in CDNi network. As referred in section 2, the URL
description for a content may be different from uCDN to uCDN and is
likely to be changed in the redirection process. An agreement to
configure a specific URL between a pair of interconnected CDN is used
in the draft [draft-ietf-cdni-framework-00], however this method is
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not flexible enough for supporting de-duplication in complex CDNi
network. So a feasible proposal is to specify a mechanism for CDNi
network to guarantee that CSP's same contents cached in different
uCDNs are identified by the same URL and that the URL is unchanged or
at least the path part remains the same in the redirection process.
The main problem of this mechanism is lack of resilience and we
prefer an alternative mechanism as introduced in section 4.2 below.
4.2. Content Naming Mechanism
This section provides a detailed description of CDNi content naming
mechanism using CDNi Metadata Protocol.
In general, CSP's content as well as its copies cached in
interconnected CDNs are delivered to numerous End-Users. The draft
[draft-ietf-cdni-framework-00] assigns each copy an identifier in the
form of an URL that is embedded with "CDN-Domain" which is used to
distinguish whether a download request is from an end-user or from an
uCDN. We use the term Resource Identifier to represent the storage
pointing to the content copies in interconnected CDNs. However,
unlike the usage in the draft [draft-ietf-cdni-framework-00], in this
document CDNi content naming mechanism specifies Resource Identifiers
as the one which is only related to contents in uCDNs. Taking the
example in section 2.3, we use Resource Identifier A to point to
content originated through uCDN A.
Although the Resource Identifier is able to identify a content, the
uniqueness of content identification can't be guaranteed, as Resource
Identifier is used to identify the storage of the content in the uCDN
and therefore will be changed during redirection processes between
different uCDNs. In order to resolve it, we introduce the term
Content Identifier that is assigned to associate with Resource
Identifier to uniquely identify the content and is similar to the URN
usage. Note that the Content Identifier MUST be globally unique.
Figure 4 shows a metadata model that can be used for maintaining the
relationship between the two types of Identifiers. Using this model,
the dCDN is able to uniquely identify and route requests towards the
same targeted content.
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+----------------------+
| Content Identifier |
+----------------------+
/ | \
/ | \
+--------------+ +--------------+ +--------------+
| Resouce | | Resouce | | Resouce |
| Indentifier 1| | Indentifier 2| ... | Indentifier n|
+--------------+ +--------------+ +--------------+
Figure 4 Metadata Model for Maintaining Relationship among Multi-IDs
Note: We need to develop an authoritative 'Entity' for creating and
maintaining the Content Identifier.
4.3. Description of Content De-duplication
In this section the details of the solutions that use CDNi Content
Naming mechanism for content de-duplication are discussed.
By using the content identification model included in content
metadata, an interconnected CDN is able to detect content repetition.
The content status must be synchronously updated by the
interconnected CDN. According to content status, the interconnected
CDN can determine whether the resource copy is cached or not.
We present several procedures for optimized implementation of
avoidance of CDNi content repetition.
4.3.1. Pre-Positioned Content Acquisition
The following flow illustrates how the two uCDNs successively pre-
position the same content in the dCDN. In this flow, the content to
be pre-positioned in the dCDN is identified by different Resource
Identifiers corresponding to uCDN A and uCDN B.
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+--------+ +--------+ +--------+
| dCDN | | uCDN A | | uCDN B |
+--------+ +--------+ +--------+
| Pre-position Request | |
|<-------------------------| |
+--------------+ | |
| Detection of | | |
| content rep- | | |(1)
| etition | | |
+--------------+ | |
| OK | |
|------------------------->| |
| | |
| Acquisition Request | |
|------------------------->| |
| | |
| Content Data | |(2)
|<-------------------------| |
+--------------+ | |
| Update cont- | | |
| ent status | | |
+--------------+ | |
| Pre-position Request |
|<-----------------------------------------------------|
+--------------+ | |
| Detection of | | |
| content rep- | | |(3)
| etition | | |
+--------------+ | |
| OK |
|----------------------------------------------------->|
| | |
Figure 5 Acquisition of Pre-Positioned Content
The steps that are illustrated in the figure are as follows:
1. The uCDN A requests that the dCDN pre-positions a particular
content item identified by its Resource Identifier and Content
Identifier. Receiving the message, the dCDN uses the Content
Identifier to look up target metadata to see whether the same content
item is cached. With the result that the metadata does not exist,
the dCDN replies to uCDN A with an OK message to notify that no such
copy has been cached and content pre-position is required.
2. The dCDN acquires the content from uCDN A. Once the content is
pre-positioned, dCDN updates the content status and maintains the
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binding relation between Resource Identifier and Content Identifier
metadata.
3. The uCDN B requests that dCDN pre-positions the same content item
identified by its Resource Identifier and Content Identifier.
Receiving the message, the dCDN uses the Content Identifier to look
up target metadata. Because such metadata exists, dCDN determines
that the content is already cached. Then, dCDN replies to uCDN A
with an OK message to notify that the same copy has been cached and
the pre-position request should be cancelled. The dCDN locally binds
the new Resource Identifier provided by uCDN B with the Content
Identifier.
4.3.2. Dynamic Content Acquisition
The following flows illustrates how the dCDN performs content de-
duplication in cases of a cache miss and a cache hit without content
pre-positioning.
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+----------+ +------+ +------+
| end-user | | dCDN | | uCDN |
+----------+ +------+ +------+
| Content Request | |
|------------------------------------------------------->|
| Content Redirection | |
|<-------------------------------------------------------|(1)
| Content Request | |
|-------------------------->| |
| | Content id Acquisition |
| |<-------------------------->|
| +--------------+ |
| | Detection of | |
| | content rep- | |(2)
| | etition | |
| +--------------+ |
| | Acquisition Request |
| |--------------------------->|
| | Content Data |
| |<---------------------------|
| +--------------+ |(3)
| | Update cont- | |
| | ent status | |
| +--------------+ |
| Content Data | |
|<--------------------------| |
| | |
Figure 6 Dynamic Content Acquisition (cache miss case)
The steps that are illustrated in the figure are as follows:
1. A content request originated from an end-user is received at
uCDN. The uCDN processes the request and recognizes that the end-
user is best served by a dCDN. So uCDN redirects the request to the
dCDN by sending redirection response to the end-user who then
requests the content from the dCDN. The uCDN encapsulates Resource
Identifier of the requested content item in the redirection response.
2. Receiving the request, the dCDN uses the Resource Identifier
pointing to the requested resource to fetch the corresponding Content
Identifier from the uCDN. The dCDN then uses the Content Identifier
to look up target metadata to check whether the content item is
cached. With the result that such metadata does not exist, the case
of a cache miss is determined by the dCDN, and therefore content
needs to be downloaded from the uCDN before delivered to the end-
user.
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3. The dCDN acquires the requested content from uCDN A. Once the
content is cached, dCDN updates the content status and maintains the
binding relation between Resource Identifier and the Content
Identifier metadata. The dCDN then delivers content data to the end-
user.
+----------+ +------+ +------+
| end-user | | dCDN | | uCDN |
+----------+ +------+ +------+
| Content Request | |
|------------------------------------------------------->|
| Content Redirection | |
|<-------------------------------------------------------|(1)
| Content Request | |
|-------------------------->| |
| | Content id Acquisition |
| |<-------------------------->|
| +--------------+ |
| | Detection of | |
| | content rep- | |(2)
| | etition | |
| +--------------+ |
| Content Data | |
|<--------------------------| |
| | |
Figure 7 Dynamic Content Acquisition (cache hit case)
The steps that are illustrated in the figure are as follows:
Steps 1 and 2 are exactly the same as steps 1 and 2 of Figure 6,
except that this time dCDN determines the case of a cache hit
according to the existence of such record in the corresponding
Content Identifier metadata.
This flow differs from the one in Figure 6 only in terms of not
triggering dynamic content acquisition (step 3), since the content
has already been cached by dCDN.
4.3.3. Content Removal
The following flow illustrates how the dCDN removes the content
resource under the control of the uCDN.
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+----------+ +------+ +------+
| end-user | | dCDN | | uCDN |
+----------+ +------+ +------+
| | Content Removal |
| |<---------------------------|
| +--------------+ |
| | Removal of | |
| | content | |(1)
| +--------------+ |
| | |
| +--------------+ |
| | Removal of | |
| | conjunction | |
| | metadata | |(2)
| +--------------+ |
| | OK |
| |--------------------------->|
| | |
Figure 8 Removal of Content
A premise is that the content copy to be removed has already been
cached in the dCDN from the uCDN. The steps illustrated in the
figure are as follows:
1. The uCDN requests that the dCDN removes some content resource
identified by the Content Identifier due to the deployment policy or
expiration of content!_s life-time. The dCDN then removes the
content resource accordingly.
2. Once the content resource is removed, the dCDN updates the
content status and removes the Content Identifier metadata. It then
replies a OK response to the uCDN.
5. Security Considerations
To be added later
6. IANA Considerations
This document has no IANA Considerations.
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7. Acknowledgments
To be added later
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[I-D.ietf-cdni-framework]
Peterson, L. and B. Davie, "Framework for CDN
Interconnection", April 2012.
[I-D.ietf-cdni-problem-statement]
Niven-Jenkins, B., Faucheur, F., and N. Bitar, "Content
Distribution Network Interconnection (CDNI) Problem
Statement", May 2012.
[I-D.ietf-cdni-requirements]
Leung, K. and Y. Lee, "Content Distribution Network
Interconnection (CDNI) Requirements", December 2011.
[I-D.narten-iana-considerations-rfc2434bis]
Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs",
draft-narten-iana-considerations-rfc2434bis-09 (work in
progress), March 2008.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
July 2003.
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Authors' Addresses
WeiYi Jin
ZTE Corporation
Nanjing, 210012
China
Phone: +86 025-52871364
Email: jin.weiyi@zte.com.cn
Mian Li
ZTE Corporation
Nanjing, 210012
China
Phone: +86 025-88014641
Email: li.mian@zte.com.cn
Bhumip Khasnabish
ZTE Corporation
New Jersey, 07960
USA
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Jin, et al. Expires December 21, 2012 [Page 17]